1. Field of the Invention
This invention relates to railroad drawbridges, and more particularly to rail finders for such drawbridges.
2. Prior Art
Rail finders indicate when the rails of a raised or drawn aside railroad drawbridge have returned to their operative position in which they are aligned with the rails of the tracks leading to and from the railroad bridge. Today's railfinders incorporate switch boxes having a rotary shaft. The switch-box rotary shaft is rotated through an extended linkage and lever arrangement actuated by the raising and lowering of the adjacent rail of the bridge. A weighted lever pivotally mounted outside the tracks but extending underneath the adjacent rail, is engaged by the adjacent rail when a bridge descends and held in a horizontal position. The weighted lever is adjustably fixed at a midpoint to a shaft extending parallel to the rail and adjustably mounting a second lever extending generally horizontally away from the tracks. As a bridge is opened and the weighted lever follows its rising rail, the second lever swings downward to pull on one end of a turnbuckle link whose other end is connected to a lever adjustably depending from the rotatable shaft of a switch box.
The extended linkage, unfortunately, is the source of many false signals. The lost motion inherent in such an extended linkage enables its parts to be freely vibrated, not only from the movement of the heavy bridge parts but also from the passage of the trains. These vibrations shake the switch box with the result that contacts therein are closed or opened and provide false signals of the bridge's condition.
The extended linkage is also difficult and expensive to install; in fact a high level of skill is required. Not only must the parts be connected so as to send the correct signals for all conditions of the bridge, but the parts must also be so positioned as to operate under all weather conditions. For example, operation in or under snow and ice must be facilitated. Expansion and contraction due to the extremes of heat and cold must be accommodated.
The problems of the extended linkage are compounded by the switch box locations mandated by the extended linkage. Because of the length of the extended linkage, the switch boxes (and the extended linkage) must be mounted outside the tracks. This places them in the path of frequently hanging third rail shoes of electrically powered trains. Since the switch or circuit controlling box is of a cast metal, it usually cannot withstand even one strike from a a shoe. Even the extended linkage levers and arms receive damage due to the force of some hits. Even if the very expensive switch box, and/or elements of the extended linkage need not be replaced, expensive internal repair and/or adjustment due to the strikes or blows to the circuit boxes by the hanging shoes may still be necessary. Of course, even more crucial, is the impairment of rail traffic, particulary morning commuter traffic.